The drilling of boreholes in rock, ore, coal and concrete (hereinafter "rock"), is a task performed during the course of operations common to the mining, construction and petroleum industries. Typical uses for boreholes include placement of explosives, placement of rock support pins and tapping deposits of natural gas and petroleum. Typical drilling machines produce boreholes in rock by pushing an elongated hollow tool stem (drill rod, drill steel, drill pipe) having a workface with hardened cutters (drill bit) against the rock while applying rotating and/or impacting forces to the drill bit. The cutter edges on the drill bit break particles of rock and scrape them away, enabling the drill bit to advance progressively into the rock, creating a borehole. The rock particles are normally flushed away from the workface and out of the borehole around the advancing drill rod by means of a fluid (usually water) pumped through the drill rod and emitted near the workface through fluid passages in the drill bit. The velocity of the fluid exiting through the drill bit passages is normally less than 800 feet per second (244 m/sec) and the passages normally have a diameter greater than 0.100 inch (2.54 mm). Rapid dulling of the drill bit cutter edges occurs because of the abrasiveness of the rock and the severe mechanical stress transmitted through the cutter edges into the rock. Dulling of the cutter edges substantially reduces the rate at which the borehole is advanced. As a result, many drill rods use detachable drill bits that can be easily removed from the drill rod. This makes it practical for the drilling machine operator to have a plurality of drill bits available at the work site, where they can be quickly replaced when dull and resharpened at the convenience of the operator without delaying the drilling operation.
A typical drill bit assembly consists of a short body with means of attaching it to the drill rod. The assembly further includes fluid passages which are connected to fluid passage of the drill rod. The cutters on the workface are usually constructed of hardened steel, tungsten carbide, diamond or other similarly wear resistant materials. When wear limits are reached, the entire drill bit assembly is normally scrapped.
Extensive laboratory and field tests have demonstrated that borehole drilling advance rates can be substantially improved if the drill bit cutters are assisted by high-velocity fluid cutting jets. These jets are created by increasing the fluid pressure inside the drill rod in conjunction with installing special fluid passage orifices (nozzles) in the drill bit. These nozzles create concentrated fluid streams (jets) that are directed at the borehole workface, cutting into it while it is simultaneously being attacked by the drill bit cutters. Fluid jet cutting makes it easier for the drill bit cutters to break the rock, thus increasing the borehole advance rate while reducing the rate of cutter wear. In order to achieve sufficient jet velocity to enable cutting of the rock workface, differential pressure across the nozzles in the drill bit will range from about 5,000 to 60,000 lbs./in.sup.2 (34.5 MPa-413.7 MPa) or higher depending on the hardness and type of rock encountered. Jet velocities must normally exceed about 800 feet per second and the fluid passage orifice (nozzle) diameters will normally be less than about 0.060 inches (1.5 mm), with nozzles as small as about 0.003 inch (0.07 mm) diameter sometimes used.
In order to obtain full advantage of the beneficial effects of fluid jet cutting assistance, it is often necessary to mount a plurality of nozzle orifices in a single drill bit, with the nozzles aimed at different portions of the borehole workface. By way of non-limiting example, it has been found beneficial to have four nozzle orifices in a drill bit for 1.0 inch (2.54 cm) diameter boreholes. Larger diameter holes require progressively larger numbers of nozzle orifices.
The necessity to machine multiple fluid passages and nozzle mounts into the drill bit body causes a substantial increase in the cost of manufacturing the drill bit. When wear limits are reached on the outer workface of the drill bit, the fluid passages are still servicable. However, the expensive assembly must be scrapped, as the fluid passages are integral with the bit workface.
Several attempts have been made to solve the wear problem, most of which fall into the catagories of either (a) improving the wear resistance of a single piece drill bit cutter/nozzle assembly, or (b) separating the cutter assembly from the nozzle housing. Regarding improving the wear resistance of a single piece cutter/nozzle assembly, only limited success has been achieved. Cutter life has been improved by increasing the number of cutting jets per unit of borehole diameter and by using special hardened cutter inserts protected with diamond covered surfaces. However, the total cost per increment of borehole length is still high because the drill bit body tends to wear rapidly from the erosive slurry rebound that results when high-velocity fluid jets strike abrasive rock surfaces. Many harder materials that can resist jet rebound erosion are not metalurgically or structurally compatable with high-pressure fluid passages and nozzle mounts.
Separation of the cutter assembly from the nozzle body has been previously accomplished in a number of different ways. All of the known previous attempts share three major shortcomings that clearly distinguish them from the inventive concept described and claimed herein.
In some of the prior attempts, the nozzle housings and cutter assemblies are attached together in such a way that a dull cutter assembly cannot be removed without loosening or removing the nozzle housing. This presents the opportunity for dirt particles to enter the fluid supply passages and clog the small orifices necessary to create the fluid cutting jets. Experience has shown that accidental contamination of fluid passages causing plugging of nozzle orifices is one of the most common problems with fluid jet apparatus. The disclosed concept allows the cutter assembly to be quickly removed without loosening any fluid passage connection or allowing the possibility of dirt entering the fluid passages.
Most of the prior attempts utilize a cutter assembly with a large hole(s) through which one or more jets pass. As a result, a significant portion of the nozzle housing is exposed to rapid wear from jet rebound erosion and erosion from rotating the nozzle housing while it is immersed in the rock particle slurry flowing away from the workface of the borehole. The disclosed concept has each individual jet emitted through comparatively small holes in the cutter assembly whereby the nozzle housing is completely protected from jet rebound erosion. Additionally, the cutter assembly protects the sides of the nozzle housing so that wear caused by the rock particle slurry is greatly reduced.
Some of the prior attempts use small nozzle housings that must be located at the center of the cutter assembly. As borehole diameter increases, the fluid cutting jets must travel progressively longer distances to reach the outer portions of the borehole workface. This greatly reduces cutting efficiency, due to the tendency of fluid jets to decay within a short distance of the nozzle orifice when emitted into the slurry environment present at the workface of a borehole. Additionally, the geometry of the nozzle housings limit the quantity of cutting jets that can fit into the housing. These factors make the previous attempts very inefficient for larger diameter boreholes. The disclosed concept uses multiple nozzle orifices located at a uniformly close proximity to the borehole workface all across the workface diameter, enabling much better fluid jet cutting efficiency in larger diameter boreholes.